1. Field of the Invention
The invention relates to a power supply. More particularly, the invention relates to a power supply without using an electrolytic capacitor at its input side, which may be adapted to a small-size liquid-crystal display (LCD).
2. Description of the Related Art
Referring to FIG. 1, there is shown a schematic diagram of a conventional power supply 1 adapted to a small-size LCD. The power supply 1 includes an electromagnetic interference (EMI) filter 11, a bridge rectifier 12, an electrolytic capacitor 13, and a flyback converter 14. An input voltage provided by an alternating-current (AC) power supply passes through the EMI filter 11 which is used to suppress conducted EMI noise, and then the input voltage is rectified by the bridge rectifier 12 to produce a full-wave rectified voltage. The electrolytic capacitor 13 filters the full-wave rectified voltage and stores its transferred energy to produce a stable direct-current (DC) voltage Vdc. Because the electrolytic capacitor 13 has a larger capacitance value to store more energy or have better energy storage capability, the voltage Vdc across the electrolytic capacitor 13 has much smaller ripple and is regarded as a stable DC voltage source. The DC voltage Vdc is converted by the flyback converter 14 to produce output voltages Vo1 and Vo2. The output voltage Vo1 is, for example, 12 V or 16 V which may supply power to a light-emitting diode (LED) backlight driving circuit and a display panel driving circuit of the LCD, and the output voltage Vo2 is, for example, 5 V which may supply power to a mainboard of the LCD.
The flyback converter 14 includes a converting circuit and a control circuit. The converting circuit includes a transformer T1, a power transistor Q1, diodes D1 and D2, and capacitors C1 and C2. The control circuit includes a pulse-width modulation (PWM) controller U1, resistors R1 and R2, a diode D3, capacitors C3 and C4, and an output feedback circuit FB1. To protect and limit the output power of the power supply 1 (i.e., the output power of the flyback converter 14), an over current protection (OCP) function is introduced into the control circuit of the flyback converter 14. In the embodiment, the control circuit uses the PWM controller U1 having the OCP function, for example, an EM8672 integrated circuit (IC) having 7 pins CT, COMP, CS, GND, OUT, VCC, and HV. The PWM controller U1 uses an OCP comparator CMP1 to perform the OCP function by obtaining, through the pin CS, a voltage Vr1 across the resistor R1 coupled between the power transistor Q1 and ground, and comparing the voltage Vr1 with a fixed OCP setting value Vset. When the voltage Vr1 is greater than the OCP setting value Vset, it indicates that the output power of the flyback converter 14 exceeds its rated output power, and therefore, the control circuit has to perform some action to protect the converting circuit. For example, the OCP comparator CMP1 controls a control logic circuit CTRL1 to limit a duty cycle of a PWM control signal outputted to the power transistor Q1 through the pin OUT, or to directly turn off the power transistor Q1, to achieve the OCP function and a constant output power limitation. For different rated output power applications, it only has to change the resistance value of the resistor R1 to change the rated output power limitation.
The conventional power supply 1 uses the electrolytic capacitor 13 at its input side to filter the full-wave rectified voltage produced by the rectification of the bridge rectifier 12 and store its transferred energy to produce the stable DC voltage Vdc, and therefore, under the normal operating condition, the voltage Vdc across the electrolytic capacitor 13 equals to the peak value of the input voltage of the AC power supply. Taking mains AC power supply of 220 Vrms as an example, the voltage Vdc across the electrolytic capacitor 13 is approximately 311 V. The high voltage may build up more static charges on two electrodes of the electrolytic capacitor 13, and under some condition (e.g., the abnormally increased input voltage of the AC power supply), the static charges built up on the electrodes may cause arc or spark discharge to result in the burning of fuels such as an electrolyte and a paper spacer in the electrolytic capacitor 13.